1. Field of the Invention
The present invention relates to a flow rate measuring apparatus for measuring a flow rate of a fluid which is flowing in a tank by a plurality of pumps and, in particular, to an apparatus for accurately measuring a flow rate of a coolant, by a plurality of circulation pumps, in a pressure vessel in an nuclear reactor.
2. Description of the Related Art
In a boiling-water reactor a coolant in a pressure vessel is circulated, by a plurality of circulation pumps, in the pressure vessel past a reactor core and it is necessary to accurately measure a flow rate, as well as a flow distribution, of the coolant circulated in the pressure vessel past the core and monitor the state of the reactor at all times.
A general structure of a conventional boiling-water reactor and means for measuring a flow rate of the coolant will be explained below with reference to FIG. 1. The reactor contains a pressure vessel 1 with a core 2 held therein. The core 2 is held within a shroud 6 and a steam separator 3 is arranged over the shroud 6 to separate water from a steam generated at the core and supply it as dried steam to a turbine, etc. A coolant separated from the steam flows down a passage, defined between the outer periphery of the shroud 6 and the inner wall of the pressure vessel 1, onto a location under the core 2 by a plurality of circulation pumps 10 and goes up from under the core 2 into the core where it flows out of its upper zone after being boiled. The coolant circulates in such a passage as set forth above. The circulation pumps 10 are each driven, by the corresponding motor 11 outside the pressure vessel, through the corresponding shaft 9.
In this type of reactor, it is necessary to precisely measure a flow rate of the coolant into the core and monitor the state of the reactor. A conventional means for measuring a flow rate of a coolant is so arranged as will be set forth below.
Openings 25A, 25B of pipes for a plurality of sets of core plate differential pressure gauges 25 are opened at the inlet 14 of the core 2 and located at a core support plate or at an entrance nozzle of a fuel assembly. Pressure signals corresponding to pressure at these openings are sent to a differential pressure/flow converter 26 so that a flow rate of the coolant is measured.
A plurality of pump differential pressure gauges 23 are also provided to correct and back up the core inlet differential pressure gauges 25. Openings 23A, 23B of pipes for the pump section differential pressure gauge 23 are provided at the suction and discharge sides, respectively, of the respective circulation pump 10. A differential pressure gauge 23 is inputted to a pump section calculator 24. The rotational speed of the motor 11 for driving the circulation pump 10 is measured by the corresponding speed transducer 22 and a speed signal output from the speed transducer 22 is inputted to the pump section calculator 24. With a differential pressure between the suction and discharge ports and the rotational speed of the pumps as parameters, relations between these parameters and the pump discharge are obtained in advance, using a test stand, for the respective circulation pump 10 and also have been programmed into the pump section calculator 24. The flow rate of the coolant through the pump 10 is calculated by the pump section calculator 24 and the respective circulation pump's flow rate output signal is gained from the pump section calculator 24 and inputted to a calculator 27 where a total flows rate of all the circulation pumps is obtained.
A flow rate output signal of the calculator 27 and that of the differential pressure/flow rate rate calculator 26 are fed to an operation monitor device 29 of the reactor through a correction switch 28. By the switching operation of the correction switch 28, it is possible to make a readjustment of the core plate differential pressure gauge 25 and, in the case of an functional failure of this core differential pressure line, provide a backup function.
A line of the aforementioned pump deck differential pressure gauge 23 reveals lowered accuracy in the case of a temporary stoppage, or a partial operation, of the circulation pumps 10. The reason for this will be explained below with respect to FIGS. 2A and 2B.
FIGS. 2A and 2B are modified cross-sectional views showing a pressure vessel 1 at a height level where circulation pumps 10 are located. Reference numeral 12 in FIGS. 2A and 2B shows a cylindrical support leg 12 for a shroud with leg openings 13 located in front of the respective circulation pump 10 and also located at a middle area of respective adjacent circulation pumps.
FIG. 2A shows a case in which all the circulation pumps 10 are operated with equal rotating speed with a coolant flowing in the directions indicated by open arrows in FIG. 2A. In this case, some of the discharge from the respective circulation pump 10 flows directly from the leg opening 13 which is in front of the respective circulation pump into a lower plenum and a remaining portion of the discharge from the respective circulation pump 10 horizontally flows in a circumferential direction. The latter coolant flow and a coolant flow coming from the next adjacent circulation pump meet at the middle area of both the pumps and flow into the lower plenum through the leg opening 13 which is located between the circulation pumps.
FIG. 2B shows a case in which some circulation pump (for example, a circulation pump 10B) is out of service and the other circulation pumps 10A are operated with equal rotating speed, a coolant flowing in the directions indicated by solid arrows in FIG. 2B. In this case, the discharge flowing in the circumferential direction from the neighboring pumps 10A located at both side of the idle pump 10B flow toward the discharge side of the idle pump 10B. Some of the coolant flowing toward the discharge side flows into the lower plenum through the leg opening 13 in front of the idle pump 10B and others flow backward via the idle pump 10B. Even if, in this case, the flow rate of the discharge from the individual operating pumps are summed up it is not possible to accurately calculate a flow rate of coolant through the core.
Although a simpler case has been explained above in conjunction with FIG. 2B for ease in understanding, some of circulation pumps may, in practical case, be stopped or a plurality of circulation pumps may be operated with different rotating speed. It has been difficult, in such a complicated case, to measure a flow rate of coolant precisely.
The aforementioned problem arises upon the exact measurement of a flow rate of coolant in the pressure vessel in the reactor as well as in other apparatus and chemical plants. In a heat exchanger, a boiler, a agitating apparatus in a chemical plant, a dialyzer, a gas reaction apparatus, a solid/fluid separation apparatus etc, a complex fluid flow/circulation passage is formed inside of vessel and a fluid flow through the passage. Even in these apparatuses, it is difficult to exactly measure the state of a fluid flow in the vessel in an off-normal operational condition and there is a growing demand for an apparatus of accurately measuring a flow rate of a complex fluid flow in a vessel, or a container, as in the aforementioned nuclear reactor.